It is known that a slide rheostat used for control of a tone volume has a light-emitting device installed on a knob that reciprocates on a guide member, the light-emitting device being electrically connected to a power supply for indication of the tone control position of the knob. In this prior art construction, since the light-emitting device is installed on the reciprocating knob, the electrical connection between the light-emitting device and the power supply is complex, and the knob is large. Hence, it has been difficult to make the illuminated slide rheostat thin. Another disadvantage is that the position of the knob relative to the indicator portion cannot be easily checked, because the indicator portion is illuminated by the light emanating from the light-emitting device that is carried on the knob.